Fluid Catalytic Cracking (FCC) is a commonly-used process in oil refineries that produces high yields of gasoline and liquefied petroleum gas, which are in a high demand in the United States, and throughout the world. Despite the long existence of the fluidized catalytic cracking process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e., yield and selectivity.
In general, commercial fluid catalytic cracking processes are carried out in FCC units in which the riser reactor is either internal to, or external to, a larger vessel, typically known as a disengaging vessel or reactor vessel. As known within the art, FCC units with either internal or external risers, present their own different advantages and disadvantages as related to, among other things, size and efficiencies.
Typically, in FCC processes, catalyst is brought into contact with a hydrocarbon feed in a reaction zone, which is generally in the form of an elongated tube called the riser, riser reactor or riser reactor pipe (although sometimes the reactor can be a downflow reactor). The riser can be located inside (i.e., an internal riser), or outside (i.e., an external riser) of the disengager vessel. The catalyst is then substantially separated from the hydrocarbons in one or more separation stages and the cracked hydrocarbons, accompanied by as small a quantity as possible of catalyst, leave the reaction zone for product recovery in downstream fractionation unit and further processing operations. The separated spent catalyst from the separators is collected in the bottom of the disengager (in a dense bed) where it typically is brought into contact with a gas which is different from the hydrocarbons, such as, for example, ammonia, nitrogen, or steam, to encourage removal and recovery of volatile hydrocarbons entrained with the catalyst, commonly referred to as stripping (or steam stripping where steam is used as the stripping medium). The catalyst is then evacuated to a regeneration zone where the coke formed during the reaction in the riser reactor and hydrocarbons which have not yet been desorbed during the stripping stage are burned in an oxidizing medium.
However, in order to obtain selective products and avoid over cracking the desired hydrocarbon to less desirable by-products in the reaction zone of the catalytic cracking unit, it is preferable to rapidly separate the gaseous products produced in the contact zone from the spent catalyst, including by way of a first (rough cut) separation, which although does not provide for complete separation of the spent catalyst particles from the cracked products, sufficiently removes a substantial proportion of them in a quick fashion to reduce degradation reactions.
A number of ways exist for carrying out these operations of separation/desorption and the literature is replete with devices developed for catalytic cracking processes, which are more or less effective for such different operations. And while it is relatively simple to carry out rapid separation or effective stripping, it is difficult to carry out rapid separation and effective stripping substantially simultaneously. Further, as the price of oil is ever increasing and the amount of oil available for conversion into petrochemical products becomes rarer, there is always a need in the art for more efficient rough cut catalyst separation processes in order to obtain higher yields of desirable products.
For example, U.S. Pat. Nos. 4,288,235, 4,348,364 and 4,433,984 disclose side-by-side type apparatus for rapidly separating particulate solids from a mixed phase solids-gas stream from tubular type reactors. The apparatus projects solids by centrifugal force against a bed of solids as the gas phase makes a 180° directional change to effect separation. The solids phase undergoes two 90° changes before exiting the apparatus.
Other rapid separation and stripping apparatus include U.S. Pat. No. 5,837,129, which discloses an FCC unit having an internal riser, and a ramshorn inertial type of separator at the terminal end of a riser reactor in combination with a horizontally disposed gas outlet. The horizontally disposed gas outlet facing upwardly and toward the riser reactor, or upwardly and away from the riser reactor, provide quick and efficient separation of hydrocarbon vapor product from catalyst particles.
In general, rapid separation can be effected using cyclones directly connected to an internal riser, as described in U.S. Pat. No. 5,055,177. In this system, cyclones connected to the riser are inside a disengaging vessel, which generally also encloses a second cyclone stage. The gas separated in the first stage enters the second cyclone stage for more complete separation. The catalyst is directed into the dense phase fluidized stripping bed of the disengaging vessel where steam is injected as a counter-current to the catalyst to desorb the hydrocarbons. Such hydrocarbons are then evacuated from the reactor into the upper dilute phase of the disengaging vessel and introduced into the separation system into the second cyclone stage. The fact that there are two cyclone stages, one connected to the riser carrying out primary separation, the second generally being connected to the outlet for gas from the first stage cyclones, necessitates a very large diameter for the disengaging vessel surrounding the two cyclone stages. The dilute phase of that vessel is only traveled by the gases desorbed in the stripper, or by the gases entrained by the catalyst in the solid outlets (diplegs) of the first stage. The gases from the stripping section are thus systematically exposed to a long term thermal degradation in the stripper, because if the primary cyclone functions correctly, a fairly small quantity of hydrocarbons is entrained in the dipleg of the primary cyclone towards the stripper. The volume of the disengaging vessel being large and the quantity of hydrocarbons and stripping steam being fairly small, the surface velocity of the gases in the diluted phase of the disengager vessel outside the primary cyclones is very low typically not greater than 2 feet per second (ft/s). Consequently, the evacuation time for hydrocarbons stripped or entrained in the diplegs with the catalyst will necessarily be of the order of several minutes.
A further disadvantage of that separation system is that it introduces hydrocarbons entrained or adsorbed onto the catalyst in localized fashion into the fluidized stripping bed. Because the fluidized bed is a poor radial mixer but a very good axial mixer, there is an inevitable loss of efficiency in the stripping zone. It would be possible to improve stripping by introducing stripping gases directly into the solid outlet. Nevertheless, this would only be effective if the catalyst flowed slowly in the cyclone outlet in order not to entrain gases, which is not possible to achieve if proper operation of the primary cyclones is to be retained.
U.S. Pat. No. 6,296,812 provides an apparatus for separating and stripping a mixture of gas and a stream of particles in an upflow and/or downflow internal riser reactor. The apparatus has a reaction envelope containing a vessel for separating the particles from the mixture and a vessel for stripping the separated particles located below the separation vessel, which has a plurality of separation chambers and a plurality of stripping chambers distributed axially about one extremity of a internal riser reactor of elongate form. The upper portion of each separation chamber includes an inlet opening communicating with the reactor, so as to separate the particles from the gaseous mixture in a substantially vertical plane, with each separation chamber containing two substantially vertical lateral walls that are also the walls of the circulation chamber.
The present applicants have inventively developed a highly compact riser separation system having an external riser utilizing the concept described in U.S. Pat. No. 6,296,812, which enables proficient separation efficiency, simultaneous effective stripping and rapid evacuation of the separated hydrocarbons due to the improved compactness of the equipment while retaining all the advantages associated with the separation system in U.S. Pat. No. 6,296,812.